Method for obtaining monodisperse tabular grains

ABSTRACT

The present invention relates to a method for preparing a photographic tabular silver halide grains emulsion. The method is characterized in that the twinned silver halide seeds are precipitated, either in an external static nucleator working in laminar flow regime with a Reynolds number less than 2100, or in a kettle with a very low stirring rate with respect to the one generally used, and in that the concentration of the Ag +  ion solution is ranging from 0.04 to 0.3M. 
     Obtained are tabular silver halide grains, the diameter distribution of which is less than 15%.

FIELD OF THE INVENTION

The present invention relates to a method for preparing a photographicemulsion containing gelatin and tabular silver halide grains exhibitinga narrow size distribution.

DESCRIPTION OF THE PRIOR ART

The tabular silver halide grains, their preparation methods and theiruse have been extensively studied these last years, and they are used incommercial products. By "tabular grain" is meant a grain defined by twoparallel or substantially parallel crystalline faces, each exhibiting anotably greater surface than any other crystal face forming the grain.The aspect ratio, i.e., the diameter/thickness ratio is more than atleast 2:1, and preferably, more than at least 5:1. The diameter isdefined as being the diameter of an equivalent circle obtained from thegrain projected area, as viewed in a photomicrograph or in an electronmicrograph of an emulsion sample.

The advantages of these grains are well known: they provide a betterimage sharpness, a higher covering power, a better relationship betweensensitivity and granularity, a better separation between the blue andthe minus blue, and allow to use lower silver coverages and thinneremulsion layers.

Numerous methods for preparing the tabular silver halide emulsions havebeen disclosed. For example, U.S. Pat. No. 4,434,226 discloses tabularhalide grains having a thickness less than 0.5 μm, a diameter of atleast 0.6 μm, an average aspect ratio more than 8:1 and representing atleast 50% of the total grain projected area. These grains are preparedby a double jet method at a pBr ranging between 0.6 and 1.6.

By this method, tabular silver halide grains exhibiting a wide sizedistribution are obtained.

However, it would be highly desirable to provide a method for preparingmonodisperse tabular silver halide grains, i.e., exhibiting a narrowsize distribution. The advantages due to the narrow size distributionsare well known, the number of photographically useful grains isincreased, the sensitization can be more easily controlled since thegrains exhibit similar sizes, the contrast and the granularity of theresulting photographic element are improved.

In the present description, the dispersity is, unless otherwise stated,represented by the coefficient of variation of the diameter (COV), whichis the ratio between the diameter standard deviation of the grains andthe mean diameter of these grains. The cited values refer to measuresperformed on electron micrographs of the grains.

French Patent No. 2,534,036 discloses a method for preparing hexagonaland triangular monodisperse flat grains having a thickness less than 0.3μm, an aspect ratio of at least 5:1 and representing at least 97% of thetotal grain projected area. The coefficient of variation expressed as %of grains versus grain diameter varies between 15 and 28.4% in theexamples. This method consists in precipitating fine grains having adiameter less than 0.15 μm and letting them undergo a physical ripeningat a pAg ranging from 8.4 to 11, without any complexing agent.

U.S. Pat. No. 4,775,617 discloses a method for preparing monodisperseflat grains having a thickness ranging between 0.5 and 6 μm, an aspectratio ranging between 5:1 and 30:1 and a coefficient of variation (COV)of at least 20%, the tabular grains forming at least 50% of the totalgrain projected area. The method consists in growing the grains bycontrolling the concentration flowrate of the halide and silver ionsolutions at 50-60% of the crystal critical growth rate.

U.S. Pat. No. 4,722,886 discloses a method for preparing tabular grainshaving a thickness ranging between 0.05 and 0.5 μm, an average grainvolume ranging between 0.05 and 1 μm³ and an aspect ratio more than 2:1.The emulsion predominantly contains tabular grains. The method includesseveral steps, the precipitation being performed in presence of ammonia,which is then neutralized before the ripening and the growth. Thedispersity is calculated in volume, which is not really significativefor flat grains, in the absence of data on the grain thicknessvariations.

German Patent No. 3,707,135 discloses a method for preparingmonodisperse tabular grains having a grain size ranging between 0.2 and3 μm, an aspect ratio ranging between 2.5:1 and 20:1. The projected areaof hexagonal tabular grains is at least 70% of the total projected area.The coefficient of variation (COV) does not exceed 20% and preferably,is less than 15%. In this method, the nucleation temperature is reducedin order to obtain only hexagonal grains, without triangles.

According to these prior art patents, it can be seen that it is verydifficult to obtain tabular grains having a coefficient of variationless than 15% and which represent at the same time up to 99% of theprojected area. Either a significant proportion of tabular grains can beobtained (up to 99% of the projected area) but the variation coefficientis high, or a low variation coefficient can be obtained, but there arefew tabular grains. On the other hand, the methods for preparing silverhalide emulsions which can be used on an industrial scale must exhibitspecific characteristics, particularly for the speed and thereproducibility, which allow to reduce the cost thereof. This is thereason why there is a constant need for more performing methods in orderto manufacture monodisperse tabular silver halide emulsions.

SUMMARY OF THE INVENTION

According to the present invention, these objects can be achieved with amethod consisting in

(a) precipitating twinned silver halide seeds from silver nitrate andhalide solutions, in a precipitation medium, exhibiting a laminar flowregime, the concentration of the silver nitrate solution ranging between0.04 and 0.3M, and the seeds being received in a receiving medium;

(b) ripening the seeds by stopping the addition of reagents, understrong stirring, during 1 to 90 mn, and preferably during 20 to 30 mn,at a VAg more than 0 mV and preferably more than 20 mV;

(c) growing the grains by a double jet technique under strong stirring,at a VAg more than +10 mV and preferably more than 20 mV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a nucleator where the solutions are fed in parallel inthe same direction.

FIG. 2 represents a T-shaped nucleator where the solutions are fed inparallel in opposite directions.

FIG. 3 represents a Y-shaped nucleator where the solutions are fed withan angle of 45°.

FIG. 4 represents a nucleator where the solutions are fed in oppositedirections as in FIG. 2 and the ejection is provided through holesarranged as a crown.

FIG. 5 is an electron micrograph of the emulsion obtained in Example 5according to the invention at a 11,500× magnification.

FIG. 6 is the sensitometric curve obtained with the emulsion of theinvention and a control emulsion having a wider size distribution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The main characteristics of the present method are the flow regime andthe low concentration of the silver salt solution used to prepare thesilver halide seeds during the first step.

In the process according to the invention, a continuous externalnucleator is used, wherein the Ag⁺ ion and halide solutions arrivingseparately in a continuous flow, are mixed in a laminar, non turbulentway, the very low stirring being defined by a Reynolds number less than2100. The seeds formed are then directed towards the main kettle wherethe second step, or ripening, and then the third step, or growth, willoccur. Devices allowing to carry out this first embodiment of the methodaccording to the invention will be described below.

The seeds are then allowed to wait under the ripening conditions whilevigorously stirring, before carrying out the growth. By way of example,the stirring rate of the ripening (b) and growth (c) steps is rangingbetween 4000 and 5000 rpm for a 20 l kettle.

The present invention relates to a tabular silver halide emulsionprepared by the above disclosed method, this emulsion beingcharacterized in that it contains at least 60% of tabular grains withrespect to the total grain number and in that the diameter variationcoefficient is less than 15% and preferably, less than 10%.

The following description illustrates the external static nucleatorsallowing to carry out the first embodiment of the invention. FIGS. 1 to4 are schematic drawings of useful static nucleators.

The nucleator of FIG. 1 is a reactor exhibiting a cylindrical symmetrywherein the halide and silver nitrate solutions are fed in parallel, sothat the precipitation reaction mainly occurs at the interface of thesefluids. The conical portion located in front of the nucleator allows thefluid to be accelerated. The position of the central cone is adjustableto allow to alter the ejection rate of the fluids out of the nucleator.A tube exhibiting a variable length, not shown here, extends thenucleator.

The device of FIG. 2 is a "T"-shaped reactor, wherein the fluids are fedin parallel in opposed directions and are ejected perpendicularly tothis direction. During the ejection, the fluids can meet a portionhaving a lower diameter, allowing to adjust the ejection rate andpossibly, to be subjected to a low stirring.

The "Y"-shaped device of FIG. 3 is similar, in its principle, to thedevice of FIG. 2 but differs in that the fluids are no more fed inparallel but with an angle of 45°.

The device of FIG. 4 is similar to the device of FIG. 2 as regards theintroduction of the reagents, but the ejection is provided by one orseveral rows of holes arranged as a crown, all the holes having the samediameter and forming at least an angle of 15°.

In addition to their shape, the above disclosed nucleators contribute todetermine an average residence time of the seeds, characterized by themean duration spent by a fluid element of the Ag nitrate solutionbetween the time it is contacted by the halide salt solution and thetime it is ejected as silver halide seeds. This average residence timedepends also on the introduction flowrates of the fluids into thenucleator.

The total duration of step (a) including the formation of the twinnedseeds varies between 10 and 300 seconds.

In any case, the solution containing the seeds is directed towards agelatin solution, referenced below as a "receiving medium".

Without being bonded by a particular theory, it is thought that theclaimed nucleation conditions, i.e., the flow regime and the lowconcentration of silver salt, allow to obtain homodisperse twinnedseeds, among other seeds. It was observed that these homodispersetwinned seeds were obtained whatever the nucleation VAg may be, which ison principle only a mean VAg calculated according to the concentrationof the halide and silver salt solutions, and to the flowrate of thesesolutions. These seeds are all the more homodisperse since thephysicochemical conditions of the receiving medium of these seeds,reduce the possibility of a rapid ripening of the grains. Theseconditions are generally met if the VAg of the receiving medium is highenough, i.e., more than -10 mV, for example 30 mV, in which case thenucleation duration does not significantly affect the dispersity of thetwinned seeds. In the case where the physico-chemical conditions of thereceiving medium promote the ripening, the extension of the nucleationduration will contribute to increase the dispersity of the twinnedseeds.

The second step, or ripening, allows to remove all the non twinned seedsfor the benefit of the twinned seeds. The ripening conditions must becarefully controlled in order not to destroy the initial homodispersityof the seeds, and particularly the VAg and the ripening duration. Alower VAg provides a more effective ripening, however, the duration mustbe reduced. If the ripening duration is too long, the tabular grainsbegin to destroy themselves and the homodispersity is lost. The optimumripening conditions providing the best % tabular grains/COV ratio can bedetermined, these conditions can vary according to the nucleationconditions. In practice, the ripening VAg is more than 0 mV, andpreferably more than 20 mV. The ripening duration can vary for examplebetween 20 and 30 minutes for a ripening VAg of more than 20 mV.

In the case of the laminar external nucleator, the tube length betweenthe location where the solutions flows come into contact and where theseeds are forming and the location where the seeds arrive into thereactor, must be taken into account. Indeed, the tube must be consideredas a reactor wherein the ripening occurs all the more rapidly since theseeds are small. The length of the tube coupled to the introductionflowrates of the reagents determines an average residence time in thenucleator that can be varied from 0.5 ms to 20 ms, according to thenucleators used.

It is also possible to precede the ripening step by a phase during whichthe VAg is decreased rapidly to values less than -15 mV by adding aconcentrated silver bromide solution, followed at least one minute laterby a gelatin dump coupled to a temperature increase, allowing toincrease the VAg to the value used for the ripening. This step allows topossibly retwin a few seeds which would not have been twinned duringnucleation.

After the ripening, the double jet growth is carried out with silversalt and halide solutions having concentrations ranging from 0.5M to 4M,while vigorously stirring, at a temperature ranging from 35° to 70° C.,and with a flowrate profile which must be controlled in order to avoidrenucleation, but must be close to the critical growth rate.

The growth VAg must be more than +10 mV, and preferably more than 20 mV,in order to preserve the initial homodispersity of the twinned seeds.

The tabular silver halide grains according to the invention can besilver bromide or silver bromoiodide grains. In general, they look likeregular or irregular hexagons. FIG. 5 is a photomicrograph of emulsionsprepared according to the invention. It can be seen that these emulsionsare very homodisperse and contain only a small amount of small threedimensional grains

Modifying agents can be present during the seed precipitation, eitherinitially in the reactor, or added at the same time with one or moresalts, according to the conventional methods. These modifying agents canbe metal compounds such as copper, thallium, bismuth, cadmium, zinc,middle chalcogens (i.e., sulphur, selenium and tellurium), gold andnoble metals of the Group VIII, according to the indications mentionedin U.S. Pat. Nos. 1,195,432, 1,951, 933, 2,448,060, 2,628,167,2,950,972, 3,488,709, 3,737,313, 3,772,031, 4,269,927 and in ResearchDisclosure, volume 134, June 1975, publication 13452. ResearchDisclosure and its predecessor Product Licensing Index are published byIndustrial Opportunities Limited; Homewell, Havant; Hampshire, PO9 1EF,Great Britain.

During the third step (growth), the bromide and the silver salt can beadded to the reactor by tubes, the output of which is located at orunder the surface, by feeding by gravity or by means of apparatus whichallow to regulate the addition rate as well as the pH and/or the pAg ofthe reactor content, such as disclosed in U.S. Pat. Nos. 3,821,002 and3,031,304 and by Claes and al in Photographische Korres pondenz, volume102, No. 10, 1967, page 162. In order to obtain a rapid distribution ofthe reagents in the reactor, especially designed blending devices suchas those disclosed in U.S. Pat. Nos. 2,996,287, 3,342,605, 3,415,650,3,785,777, 4,147,551 and 4,171,224, in the British Pat. No. 2,022,431A,in the German Patent Applications 2,555,364 and 2,556,885 and inResearch Disclosure, volume 166, February 1978, publication 16662, canbe used.

In order to form emulsions according to the invention, a peptizerconcentration ranging from 0.2 to about 10% in weight based on the totalweight of the constituents of the emulsion in the reactor can be used.It is preferred to maintain the peptizer concentration in the reactor ata value less than about 6% of the total weight, before and during theseed formation and preferably, also during the subsequent ripening andto adjust later, at higher values, the vehicle concentration of theemulsion (the vehicle including the binder and the peptizer) by addingfurther vehicle amounts, in order to obtain the optimum coatingcharacteristics. The emulsion initially formed can contain 5 to 50 gabout (and preferably 10 to 30 g) of peptizer per mole of silverbromide. Subsequently, further vehicle amounts can be added to increasethe concentration up to 1000 g per mole of silver bromide.

Advantageously, the vehicle concentration in the finished emulsion ismore than 50 g per mole of silver bromide. Once applied and dried in aphotographic element, the vehicle represents about 30 to 70% of theweight of the emulsion layer.

Vehicles, both including binders and peptizers, can be chosen among thematerials commonly used as vehicles in the silver halide emulsions. Thepreferred peptizers are the hydrophilic colloids which can be used aloneor associated with hydrophobic materials. The suitable hydrophilicvehicles include materials such as proteins, protein derivatives,cellulose derivatives, for example, cellulose esters, gelatin such asalkali-treated gelatin (bone or skin gelatin) or acid-treated gelatin(pigskin gelatin), gelatin derivatives such as acetylated or phthalatedgelatin. These materials as well as other vehicles are disclosed inResearch Disclosure, volume 176, December 1978, publication 17643,section IX.

The vehicles can be hardened such as disclosed in paragraph X. Thetabular grain emulsions can be mixed with conventional emulsions, suchas disclosed in paragraph I.

The tabular grains can be chemically sensitized, such as disclosed inparagraph III and/or spectrally sensitized or desensitized such asdisclosed in paragraph IV. The photographic elements can containbrighteners, anti-foggants, stabilizers, absorbing or diffusing agents,coating aids, plasticizers, lubricants and matting agents, such asdisclosed in paragraphs V, VI, VIII, XI, XII and XVI. Methods forincorporating addenda, coating and drying, such as those disclosed inparagraphs XIV and XV, can be used. Conventional photographic supportssuch as those disclosed in paragraph XVII can be used. The resultingphotographic elements can be used for black and white photography or forcolor photography, and they form silver images and/or dye images byselective destruction, formation or physical removal of dyes, such asdisclosed in paragraph VII. The preferred color photographic elementsare those which form dye images by using color developing agents and dyeforming couplers. These photographic elements can be conventionallyexposed, such as disclosed in paragraph XVIII, and then processed suchas disclosed in paragraph XIX.

The following examples illustrate the invention.

EXAMPLES 1 TO 6 AND COMPARATIVE EXAMPLES A TO D

These examples show the influence of the different parameters claimed inthe first embodiment of the invention. The external static nucleator ofFIG. 1 or the one of FIG. 2 is used (see table).

The nucleator is coupled to a 20 l kettle containing the receivingmedium comprised of 6 liters of a gelatin solution at 30 g/l containinglg/l of sodium bromide, having a pH of 5.5, a VAg of +30 mV, and atemperature of 70° C., stirred at 4500 rpm.

A 0.5 % gelatin solution containing 0,2 mole/l of potassium bromide, ata flowrate of 108 ml/mn, and a solution containing 0.1 mole/l of silvernitrate at a flowrate of 100 ml/mn, are fed into the nucleator, bothsolutions having a pH of 5.5 and a temperature of 35° C., during 90s.The mean VAg calculated at the output of the nucleator is -38 mV.

The solution obtained in the kettle is allowed to wait in order to carryout the ripening during 20 to 30 mn, at a temperature of 70° C. and a pHof 5.5 and a stirring of 4500 rpm, at a VAg of 23 mV.

The seeds are then allowed to grow by incorporating 1.5M potassiumbromide and 1.5M silver nitrate solutions into the kettle, at 70° C. anda stirring of 4500 rpm, with increasing flowrates. The initial bromideflowrate is 8.3 ml/mn, and is increased according to a law of the type:flowrate=A+Bt.sup.α (A=8.3, B=0.439, α=1.37) up to 108.9 ml/mn. Theinitial silver salt flowrate is 8.3 ml/mn and is increased according toa law of the type: flowrate=A+Bt.sup.α (A=8.3, B=0.387, α=1.4) up to106.8 ml/mn.

The growth duration is 52 mn.

The data are collected in Table I below. "COV" is the coefficient ofvariation of the diameter, ECD is the mean circular diameter in μm, and% T is the percentage of tabular grains with respect to the total numberof grains. This percentage is more representative of the tabular grainamount obtained than the projected area generally used. Indeed, thenon-tabular grain amount as well as the average size of these grainsbeing low, it results that the area projected by the tabular grains isalways very significant (>90%) and is not representative of theprecipitation quality determined according to the objectives of thepresent invention.

It can be seen that comparative examples A and B provide a good COV(equal or less than 15%), but very few tabular grains, whereascomparative examples C and D provide a higher number of tabular grains,but a poor COV.

Examples 1 to 6 according to the invention both provide a tabular grainpercentage of more than 60% and a COV less than 15%.

FIG. 5 is an electron photomicrograph at 11 500× magnification of theemulsion of Example 5.

                                      TABLE I                                     __________________________________________________________________________    Nucleation      Ripening        Growth                                                                              Data                                    Examples                                                                           Nucleator  Ag.sup.+  Conc.                                                                     VAg  Duration                                                                           VAg   % T ECD COV                             __________________________________________________________________________    Comp. A                                                                            Laminar (FIG. 1)                                                                         0.02M +23 mV                                                                             20 mn                                                                              +23                                                                              mV 24  1.4 14                              Ex. 1                                                                              "          0.1M  "    "    "     76  1.8 10                              Comp. B                                                                            "          0.5M  "    "    "     <15 1.6 15                              Comp. C                                                                            "T"-shaped (FIG. 2)                                                                      0.1M  "    1  mn                                                                              -40                                                                              mV 83  3.2 69                              Comp. D                                                                            "          0.1M  "    1  mn                                                                              0     83  1.7 35                              Ex. 2                                                                              "          0.1M  "    1  mn                                                                              +30                                                                              mV 62  1.1 14                              Ex. 3                                                                              "          0.1M  "    1  mn                                                                              +66                                                                              mV 75  0.5 14                              Ex. 4                                                                              Laminar (FIG. 1)                                                                         0.1M  "    1  mn                                                                              +23                                                                              mV 62  1.1 14                              Ex. 5                                                                              "          0.1M  "    20 mn                                                                              +23                                                                              mV 62  1.8  8                              Ex. 6                                                                              "          0.1M  "    30 mn                                                                              +23                                                                              mV 67  1.8  7                              __________________________________________________________________________

EXAMPLE 7--Sensitometric data

The emulsion of example 1 and a control AgBr emulsion exhibiting a widersize distribution were optimally spectrally and chemically sensitized.

These emulsions were applied on a triacetate support at 0.807 g Ag/m².The resulting element samples were exposed during under a step wedge ata light source of 5500° K. The samples were developed during 3 min 15sec at 38° C. in a C-41 developer.

The sensitometric curves showed that the contrast according to theemulsion was improved with respect to an emulsion exhibiting a widersize distribution (FIG. 6).

We claim:
 1. A method for preparing a photographic emulsion containing gelatin and tabular silver halide grains having a number of tabular silver halide grains with respect to the total number of silver halide grains more than 60% and a coefficient of variation of the diameter less than 15% characterized in that:(a) twinned silver halide seeds are precipitated from halide and silver nitrate solutions, fed into a precipitation medium, under laminar flow conditions determined by a Reynolds number less than 2100, the silver nitrate solution having a concentration ranging from 0.04 to 0.3M, and the seeds being received in a receiving medium; (b) the seeds are ripened by stopping adding the halide and silver nitrate solutions, under strong stirring, during 1 to 90 mn, and at a VAg more than 0 mV; (c) the seeds are grown by a double jet technique under strong stirring, at a VAg more than +10 mV.
 2. A method according to claim 1, wherein the receiving medium in step (a) has a VAg which is more than -10 mV.
 3. A method according to any of claims 1 or 2, wherein the seeds are ripened in step (b) during 20 to 30 minutes.
 4. A method according to claim 1, wherein the VAg in step (b) is more than 20 mV.
 5. A method according to claim 1, wherein the VAg in step (c) is more than 20 mV.
 6. A method according to claim 1, wherein step (a) is carried out in an external static nucleator.
 7. A method according to claim 6, wherein the twinned seeds reside in the external static nucleator during 0.5 ms to 20 ms. 